Calculating and sizing a current transformer
Before ordering a CT there are three calculations: the ratio, a primary current that matches the installation and the burden in VA. Ratio = rated primary / rated secondary current; a 400/5 A CT has a ratio of 80 – 200 A in the conductor gives 2.5 A at the meter. The meter multiplies readings by this CT factor unless the ratio is programmed in.
Choose the next rated value above the highest operating current; a much larger CT works mostly at part load with wider error limits.
CT factor of common ratios
| Ratio | CT factor | at 100 A in the conductor |
|---|---|---|
| [100/5 A](/stromwandler/100-5a) | 20 | 5 A |
| [400/5 A](/stromwandler/400-5a) | 80 | 1.25 A |
| [1000/5 A](/stromwandler/1000-5a) | 200 | 0.5 A |
Calculators and related pages
Recommended Products
FAQ
How do I calculate the ratio?
Rated primary current divided by rated secondary current: 400/5 A = 80.
Which primary current should I choose?
The next rated value above the highest operating current.